subdepth – Unify maths subscript height

This pack­age is based on code (posted long ago to comp.text.tex by
Don­ald Arse­neau) to equalise the height of sub­scripts in maths.
The de­fault be­haviour is to place sub­scripts slightly lower when
there is a su­per­script as well, but this can look odd in some
sit­u­a­tions.